Well, I would guess that gbs8200 upsamples and sends 1024x768 or higher resolution.
VGA2USB's framerate varies with resolution -
https://www.avw.com.au/images/Epiphan/Brochure_VGA2USB.pdf :
Update Rate (Frame rate varies depending on content.)
Resolution Frames per second
640×480 28.0
800×600 20.6
1024×768 10.0
1280×1024 5.61600×1200 4.3
1920×1200 3.1
Doesn't matter that you crop, the source resolution is still 1024 x something so your max will be 4-6 fps
also note epiphan isn't particularly great for this as it supports only RGB16 (565) color space, not RGB 24 so you get only 65k colors in the first place.
You'd get better results getting a VGA to HDMI/DVI converter and then use a dvi/hdmi capture card
ex vga to hdmi converter
https://www.ebay.com/itm/374288831868 (no idea about quality)
The cheap hdmi usb capture cards will capture and compress to h264 or mpeg2 or some compressed video and stream this compressed content to the pc so the quality may not be great. But they're literally 10-15$.
Something not noname... Ugreen has a HDMI capture card that is around $40 :
https://www.amazon.com/UGREEN-Ultra-Low-Streaming-Conferencing-Compatible/dp/B0CM9C91MK/If you're able to use a pci-e capture card, this one with 10$ starting bid (47$ buy now) may give you uncompressed raw hdmi
https://www.ebay.com/itm/156233165370Seems to be timeleak hd72B :
http://www.timeleak.com.cn/EN/Product.aspx?id=662edit : seems to be using a hardware mpeg2 compressor, and can do up to 1920x1080 30p, 60fps maybe at lower resolutions...